1. Field of the Invention
The present invention relates to an image processing system, an image processing apparatus, an image processing method and a storage medium.
2. Related Background Art
Since formerly, recorders such as, e.g. a printer, a copier and a facsimile have been so arranged as to record an image composed of dotted patterns on a recording medium such as, e.g. paper and thin plastic sheet.
Such recorders can be classified into ink-jet type, wire-dot type, thermal type and laser beam type.
An ink-jet type recorder (hereinafter, referred to as ink jet recorder also), for example, is simple in configuration and is so arranged as to discharge and fly ink droplets (recording agent) from a discharge port and stick them onto a recording medium.
Besides, as for an apparatus answering to requirements (high speed recording, high resolution, high image quality, low noise and so on) in recent years when numerous recorders have put to use, an ink jet recorder is implemented as a relatively small-sized apparatus and has become rapidly widespread.
Besides, as ink jet recorders, those using a recording head with multiple ink discharge ports integrally disposed for a purpose of improving the recording speed or the like and those with a plurality of the above recording heads corresponding to colored recording are widespread in a great number.
Furthermore, in ink jet recorders, a half tone processing method such as dither method or error dispersion method is adopted as the method of faithfully reproducing the gradation of an image information item from requirements of high resolution, high image quality or the like.
The above gradation reproducing method makes image recording of excellent gradation possible in case of sufficiently high resolution (such as above the order of 1000 dots/inch) in an ink jet recorder.
In the case of low resolution (order of 360 or 720 dots/inch) in an ink jet recorder, however, recording dots in the highlight part of an image is prominent and the rough feeling of a recording image is apt to occur due to the discontinuity of pixels.
Thus, as a method of solving the above problem, there is a way to increase the number of grades in one pixel by making recording dots themselves into multi-valued ones.
Specifically, by controlling the voltage, the pulse width or the like to be applied to a recording head, for example, a way to modulate the diameter of recording dots stuck onto a recording medium to implement a recording image of high gradation is known.
On account to the high dependence on environments, however, the above way is unstable in the diameter of recording dots and further the presence of a limit to the size of recordable minimum recording dots makes it difficult to implement a high gradation recording image.
Besides, a way to change the density of recording dots within a matrix at the size of recording dots which is kept constant (density modulating process) is considered, but this process necessitates a considerable area to raise the number of grades and consequently the resolution of a recording image never fails to worsen.
Thus, as a method of solving the above problems, there have been proposed a way to implement a high gradation recording image by shooting a plurality of ink (recording agent) droplets off at substantially the same spot toward a recording medium to thus form a single dot and at this time changing the number of ink shot droplets (multi-droplet scheme), a way to use a plurality of ink types different in density and to implement a high gradation recording image based on at least two recording dots concerning the similar colors or a method composed of a combination of these two has been proposed and put to practical use.
Thereby, in an ink jet recorder, a recording image of improved gradation property, high density and high gradation can be obtained.
On the other hand, numbers of laser-beam type recorders are utilized, for example, in the field of medical treatment.
Specifically, at first, as an image used for image diagnosis in the field of medical treatment, use has still been made of a monochromatic image, recorded monochromatically.
The reason for this is that a monochromatic image is higher in density resolution to human eyes than a colored image. Especially, in case of requiring the density resolution to a great extent, a monochromatic image is used because the monochromatic image becomes more abundant in information content recognizable for human being than any colored image.
Besides, as recording media of an image, a transparent type recording medium is known to be greater in density resolution recognizable for human being than a reflective type image.
Generally speaking, the density resolution to human eyes for a colored image is said to be on the order of 8 bits, whereas it is said to be 10 to 11 bits for a monochromatic image.
Besides, regarding medical X-ray photographs or CT/MRI images recorded to transparent recording materials (transparent media), readout to the human density resolution limit is actually possible and providing an information item for image diagnosis is known to be possible.
Thus, as the recorder for recording a high gradation monochromatic image as mentioned above, a laser-beam type recorder (hereinafter, referred to as “laser imager” also) is utilized.
This laser imager is so arranged as to irradiate laser rays modulated corresponding to an image information item onto a silver salt film and to form an image on the film by the developing treatment of the film and image are often recorded at a density resolution of 12 bits with some degree of margin allowed in considerations.
However, conventional recorders such as above-mentioned ink-jet type or laser-beam type have the following problems.
First, in an ink-jet type recorder, the ink concentration in an ink tank rose with the lapse of time owing to a gradual evaporation of the ink resolvent or other like causes and a balance of gradation in a recorded image was destructed in certain cases. Especially, in a multi-droplet type or a type of expressing the gradation of an image by using ink of similar color and different densities, a fluctuation of ink density as mentioned above brings about the destruction of balance in the gradation of a recording image, the loss of smoothness in gradation and further the formation of a pseudo contour on the recorded image, thereby resulting in damages to the image quality of the recording image in certain cases.
Besides, as recording media of record image target, using 10 kinds or more of recording media different in image quality, durability and price is considered corresponding to purposes. In this case, depending on a combination of recording medium and ink or a combination of overlapping-eject ink, the amount of ink absorption, the rate of ink absorption or the size of a record dot formed differs, so that in the gradation of a record image a jump of density had occurred or an inversion of density had occurred in certain cases.
Besides, in the case when the ratio of the density change of different density ink is not constant, there is also a problem that a jump of density has occurred in the record image.
Consequently, in conventional ink-jet type recorders, the gradation of a record image is degenerated in some cases.
On the other hand, the laser-beam type recorder (laser imager) is very expensive and application of a wet developing treatment to a film is necessary and further there were problems such as waste liquid disposal and troublesome maintenance.
Besides, a dry silver-salt type developing treatment so arranged as to carry out a developing treatment by heating in place of a wet developing treatment is also present, but even in this laser imager, there was a problem of inferior image quality in contrast to an image obtained by the wet developing treatment.
Thus, to solve these problems, a recorder of carrying out the image recording of 256 grades is proposed, for example, by use of the ink-jet type capable of 50 grades or more at 600 dpi and further by addition of error diffusion treatment.
Using this recorder, an X-ray image was actually recorded at 4096 grades in place of 256 grades to obtain a good image then.
With the above recorder, however, several problems in image quality took place in certain kinds of images to be recorded in contrast to the image quality of a record image obtained by a laser imager.
Specifically, FIG. 16 shows one example of result of recording a chest X-ray photograph on a transparent film.
As shown in FIG. 16, a contour 901 appears at the part of shoulder of a photographed body in the record image 900 (chest X-ray image on the transparent film). This contour 901 at the shoulder part does not appear on a normal X-ray photo and the shoulder part in the X-ray photo smoothly changes in density.
A contour 901 as mentioned above is referred to as “pseudo contour” in a meaning that a contour appears notwithstanding the original absence of any contour.
In addition to the contour 901, the pseudo contour appears also at a portion smoothly changing in the density of record image 900.
Besides, to prevent the pseudo contour, even if the conventional correction method is used, there are cases where a contour cannot be completely prevented.